don't restrict lxml<5, and run tests with latest 5.1.0

Fixes https://github.com/fonttools/fonttools/issues/3418

requirements.txt

@@ -16,5 +16,5 @@ pyobjc==10.1; sys_platform == "darwin"
 freetype-py==2.4.0
 uharfbuzz==0.37.3
 glyphsLib==6.6.0 # this is only required to run Tests/varLib/interpolatable_test.py
-lxml==4.9.3
+lxml==5.1.0
 sympy==1.12

setup.py

@@ -97,7 +97,7 @@ extras_require = {
     # for fontTools.misc.etree and fontTools.misc.plistlib: use lxml to
     # read/write XML files (faster/safer than built-in ElementTree)
     "lxml": [
-        "lxml >= 4.0, < 5",
+        "lxml >= 4.0",
     ],
     # for fontTools.sfnt and fontTools.woff2: to compress/uncompress
     # WOFF 1.0 and WOFF 2.0 webfonts.